Cross-linked polymers based on cyclodextrins for removing polluting agents

ABSTRACT

Cyclodextrins cross-linked by reaction with organic carbonates and the use thereof as agents capable or removing pollutants from fluids of various origin, in particular from contaminated waters.

The present invention relates to cyclodextrins cross-linked by reactionwith organic carbonates and the use thereof as agents capable ofremoving pollutants from fluids of various origin, in particular fromcontaminated waters.

The problem of water contamination by a variety of both inorganic andorganic pollutants is topical.

More particularly, ground waters and watercourses can often containorganic pollutants which have be thoroughly removed.

According to the prior art, many different alternatives may becontemplated for decontaminating waters: extraction with solvents,reverse osmosis, absorption on zeolites, absorption on activated carbon.

The use of solvents is not an environmentally acceptable process, mainlydue to retention of said solvents by the treated waters.

Zeolites are indeed much more suitable for absorbing water than organicmolecules. The use of membranes in reverse osmosis processes is a veryeffective purification technique, but high operative pressures (20-100bars) are required to overcome the hydrodynamic resistance and generallythe available membranes are not 100% selective.

Finally, activated carbon with very high surface area allows to absorb agreat number of organic compounds, but it fails to remove thosecontaminants which, albeit present in waters at extremely lowconcentrations, of the order of ppb, are still very dangerous forhealth, or molecules undesired for very specific applications. Suchtypes of widespread molecules are polychlorobiphenyls (PCB) andgenerally phthalic acid esters.

Furthermore, activated carbon tends to be deactivated in the presence ofhumidity and has to undergo controlled pyrolysis to be regenerated,which involves at least 10-15% by weight loss of material, thus makingany quantitative recovery substantially impossible. Moreover, partialcombustion might release highly toxic substances in the atmosphere, andhas therefore to be carried out in suitable plants.

Cyclodextrins (CDs) are cyclic, non-reducing oligosaccharidescharacterized by a typical toroidal cone shape. The atom arrangement inthe space is such that the inside cavity is lipophilic, while theoutside of the torus is highly hydrophilic. As a consequence, CDs areable to form stable inclusion complexes with organic molecules ofsuitable polarity and size even in aqueous solution. In the last twodecades, CDs have therefore found applications in a variety of fields inchemistry (pharmaceuticals, analytics, catalysts, cosmetics and thelike).

However, their inclusion constants are usually rather low and rarelyexceed the value of 10³. No significant improvements are attained bytransforming CDs into either soluble or insoluble polymers. Unmodifiedcyclodextrins, therefore, are not useful for removing pollutants fromaqueous solutions.

WO 98/22197 discloses cyclodextrins cross-linked with suitablediisocyanates, capable of binding organic compounds even with inclusionconstants of 10⁸-10⁹. Among the many organic compounds described,polychlorobiphenyls, phthalic acid esters and halogens are not cited.Furthermore, the production of these cross-linked cyclodextrins involvesthe use of highly toxic diisocyanates.

It has now been found that cyclodextrins cross-linked through carbonatebonds are able to strongly bind organic molecules and to remove themfrom aqueous solutions even at very low concentrations.

Therefore, the present invention relates to cross-linked cyclodextrinsobtainable by reacting a cyclodextrin with a carbonyl compound offormula X—CO—X wherein X is chlorine, imidazolyl or a —OR group in whichR is C₁-C₄ alkyl.

The reaction can be represented by the following scheme:H—O-β-CD-OH+X—CO—X→-(β-CD-OCOO-β-CD-OCOO)_(n)—wherein X has the meanings defined above and n is an integer which canrange within wide limits, depending on the conditions used in thecross-linking reaction.

The reaction is carried out in carbonyl compound excess, preferably in aX—CO—X/CD molar ratio of 4 to 16, in a suitable solvent, in particularin a polar aprotic solvent such as dimethylformamide, dimethylsulfoxideand the like, optionally in the presence of bases such as tertiaryamines. The reaction can be carried out at temperatures ranging from 10°C. to the reflux temperature of the solvent, for times ranging from 1 to48 hours.

Both natural (α, β, γ) cyclodextrins and derivatives thereof, such ashydroxypropyl-β-cyclodextrins, can be used.

Preferred carbonyl compounds are dimethyl carbonate andcarbonyl-diimidazole. Dimethyl carbonate can optionally be used at thesame time as solvent and reagent.

The cross-linked cyclodextrins of the invention are in the form ofmicro- or nano-porous material capable of absorbing with high affinitycontaminants of various type from liquid, gaseous or solid matrices, inparticular from liquid matrices such as drinking waters, industrialwastes, ground waters, water for special industrial applications (withhigh purity) and the like. The cross-linked cyclodextrins of theinvention proved to be capable of absorbing even very low amounts (suchas a few ppb) of compounds such as PCB, dioxins, halogenatedhydrocarbons (PCT, PCBT, PCDD, PCDF), aromatic optionally halogenatedhydrocarbons, phthalates or other compounds which may be generallydefined POPs (Persistent Organic Pollutants). The simple addition ofcross-linked cyclodextrins in amounts of about 10-100 mg/ml to thematrix to treat and the subsequent filtration of the solid residuedrastically decrease the content of the pollutants present in the matrixitself. Decontamination may be assisted by irradiation with ultrasounds,UV radiation and/or microwaves. Pollutants can be optionally previouslyextracted from the matrix itself. The pollutants-saturated cyclodextrinscan then be recovered by extraction with a suitable solvent.

The invention is illustrated in greater detail in the followingexamples.

EXAMPLE 1

4.54 g of anhydrous β-cyclodextrin in 100 ml of anhydrous DMF is addedwith 5.19 g of carbonyldiimidazole. The reaction proceeds at 70° C. for24 hours under magnetic stirring. After completion of the reaction, thesolution is left to cool at room temperature, then the product is addedof a large excess of bidistilled water, recovered by filtration undervacuum, washed with water and subsequently purified by prolonged Soxhletextraction with ethanol. The resulting product is dried under vacuum andground in mechanical mill to obtain a homogeneous powder.

EXAMPLE 2

1.0 g of anhydrous α-cyclodextrin in 10 ml of anhydrous DMF is addedwith 1.34 g of carbonyldiimidazole. The reaction proceeds at 70° C. for24 hours under magnetic stirring. After completion of the reaction, thesolution is left to cool at room temperature, the product is added of alarge excess of bidistilled water, then recovered by filtration undervacuum, washed with water and subsequently purified by prolonged Soxhletextraction with ethanol. The resulting product is dried under vacuum andground in a mechanical mill to obtain a homogeneous powder.Thermogravimetric analysis of the resulting polymer is reported in FIG.1.

EXAMPLE 3

1.0 g of anhydrous γ-cyclodextrin in 10 ml of anhydrous DMF is addedwith 1.0 g of carbonyldiimidazole. The reaction proceeds at 70° C. for24 hours under magnetic stirring. After completion of the reaction, thesolution is left to cool at room temperature, the product is added of alarge excess of bidistilled water, then recovered by filtration undervacuum, washed with water and subsequently purified by prolonged Soxhletextraction with ethanol. The resulting product is dried under vacuum andground in a mechanical mill to obtain a homogeneous powder.

EXAMPLE 4

3.0 g of anhydrous HP-β-cyclodextrin in 30 ml of anhydrous DMF is addedwith 3.0 g of carbonyldiimidazole. The reaction proceeds at 70° C. for24 hours under magnetic stirring. After completion of the reaction, thesolution is left to cool at room temperature, the product is added of alarge excess of bidistilled water, recovered by filtration under vacuum,washed with water and subsequently purified by prolonged Soxhletextraction with ethanol. The resulting product is dried under vacuum andground in a mechanical mill to obtain a homogeneous powder.

EXAMPLE 5

2.0 g of anhydrous β-cyclodextrin are dissolved in 30 ml of anhydrousDMF. The solution is added 1 ml of triethylamine and 14.8 ml ofdimethylcarbonate, then refluxed for 3 hours. After completion of thereaction, the solution shows increase in viscosity and the solvent isevaporated off under vacuum. The resulting polymer is purified byprolonged Soxhlet extraction. FT-IR spectrum of the resulting polymer isreported in FIG. 2.

EXAMPLE 6

3 g of anhydrous dextrin 10 are added to 30 ml of anhydrous DMF. 2.32 gof CDI are added thereto, reacting for 1 hour at 100° C. Aftercompletion of the reaction, the resulting solid is recovered, washedwith hot water, then with ethanol, dried and ground in a mechanical millto obtain a homogeneous powder.

The resulting polymers were tested for removal of organic molecules fromaqueous solutions.

EXAMPLE 7

5 ml of water containing 350 ppm of chlorobenzene was added with 100 mgof polymer of Example 2. Once reached equilibrium, the solid wasfiltered off and the solution analyzed at UV-Vis. The concentration ofresidual chlorobenzene was 52 ppm (retention ability about 15 mg/g ofresin).

EXAMPLE 8

5 ml of water containing 325 ppm of chlorobenzene was added with 100 mgof polymer of example 1. Once reached equilibrium, the solid wasfiltered off and the solution analyzed at UV-Vis. The concentration ofresidual chlorobenzene was 27 ppm.

EXAMPLE 9

5 ml of water containing 350 ppm of chlorobenzene was added with 100 mgof polymer of example 3. Once reached equilibrium, the solid was ofiltered off and the solution analyzed at UV-Vis. The concentration ofresidual chlorobenzene was 90 ppm.

EXAMPLE 10

5 ml of water containing 350 ppm of chlorobenzene was added with 100 mgof polymer of example 4. Once reached equilibrium, the solid wasfiltered off and the solution analyzed at UV-Vis. The concentration ofresidual chlorobenzene was 40 ppm.

EXAMPLE 11

5 ml of water containing 40 ppm of chlorobenzene was added with 200 mgof polymer of example 1. Once reached equilibrium, the solid wasfiltered off and the solution analyzed by GC-MS. No traces of residualchlorobenzene were detected.

EXAMPLE 12

5 ml of an aqueous solution saturated with a mixture of chlorobiphenylcongeners named Askarel was added with 100 mg of polymer of example 1.Once reached equilibrium, the solid was filtered off and the solutionanalyzed by GC-MS. No traces of residual PCB congeners were detected.

EXAMPLE 13

5 ml of an aqueous solution saturated with dibutyl phthalate (13 ppm)was added with 100 mg of polymer of example 2. Once reached equilibrium,the solid was filtered off and the solution analyzed by GC-MS. Thephthalates residual concentration was 2.6 ppm.

EXAMPLE 14

5 ml of an aqueous solution saturated with a mixture of dibutylphthalate congeners (13 ppm) was added with 500 mg of polymer of example2. Once reached equilibrium, the solid was filtered off and the solutionanalyzed by GC-MS. No traces of phthalates were detected in the aqueoussolution.

EXAMPLE 15

A chlorobenzene aqueous solution (5 ml) containing 330 ppm ofchlorobenzene was added with 100 mg of polymer of example 1. Oncereached equilibrium, the solid was recovered by filtration and immersedin ml of absolute ethanol. The alcoholic solution was removed from thesolid and analyzed at UV-Vis. Analysis showed the quantitative recoveryof chlorobenzene so that the polymer could be recycled withoutsubstantial loss of activity and capacity.

EXAMPLE 16

A chlorobenzene aqueous solution (5 ml) containing 400 ppm ofchlorobenzene was added with 100 mg of polymer of example 5. Oncereached equilibrium, the solid was filtered off and the solutionanalyzed at UV-Vis. FIG. 3 reports the chlorobenzene residualconcentrations after 30 minutes, 1 and 24 hours, from top to bottom,respectively.

1. Cross-linked cyclodextrins obtainable by reacting a cyclodextrin witha carbonyl compound of formula X—CO—X wherein X is chlorine, imidazolylor n —OR group in which R is C₁-C₄ alkyl.
 2. Cross-linked cyclodextrinsas claimed in claim 1, wherein cyclodextrins are selected from naturalcyclodextrins (a, 13, y) and the derivatives thereof.
 3. Cyclodextrinsas claimed in claim 2, wherein cyclodextrins derivatives arehydroxypropyl-cyclodextrins.
 4. Cyclodextrins as claimed in claim 1,cross-linked by reaction with dimethylcarbonate or carbonyl-diimidazole.5. Cross-linked cyclodextrins as claimed in claim 1, obtainable byreacting a cyclodextrin with a carbonyl compound excess.
 6. Cross-linkedcyclodextrins as claimed in claim 5, wherein the carbonylcompound/cyclodextrin ratio ranges from 4 to
 16. 7. A process for thedecontamination of liquid, gaseous or solid matrices, which comprisesadding said matrices with an effective amount of the cross-linkedcyclodextrins of claim 1, followed by filtration/separation of to thesolid residue.
 8. A process as claimed in claim 7, wherein the matrix iswater and the pollutants are polychlorinated biphenyls (PCBs),polychlorinated terphenyls (PCTs), PCBT, polychlorinateddibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs),Persistent Organic Pollutants (POPs).
 9. A process as claimed in claim8, wherein the cross-linked cyclodextrins are used directly on thematrix and/or indirectly through extraction of the pollutant from thematrix itself.
 10. A process as claimed in claim 7, further comprisingthe treatment of the matrices with ultrasounds, microwaves and/or UVradiation.
 11. A process for the recovery of the pollutant-saturatedcross-linked cyclodextrins of claim 1 by extraction with a suitablesolvent.